I bought a few silver age books on ebay that I suspect had this type of color touch. Not a great loss because they were $20 books... but, this goes to show you any book can be restored -- not just expensive keys.
The touch is some sort of fine black powder that is rubbed into small creases. It's hard to detect because the color rub only sticks to the creased area where the original inks are broken, so the touch is actually only as wide as the crease itself. If you look at the cover at an angle, the affected area may be a bit duller than the gloss on the rest of the cover. Also, if you gently rub your finger over the area, a tiny amount of black may rub off.
One way to detect color touch is to look for fine creases where the ink does NOT appear to be broken. Then look much closer. This is where many color touches will be found.
Regarding the ebay seller that sent me the color touched book, he replaced the first book that had a certain dry rub color touch with another one that I suspect had a smaller touch. At that point, it wasn't worth pursuing any further. I just avoid his auctions now.